1. Field of the Invention
This invention relates to a tomographic apparatus for obtaining images of the interior of a body using nuclear magnetic resonance (NMR) phenomenon, i.e. so-called NMR imaging apparatus, and more particularly to an NMR imaging apparatus permitting observation of enlarged images of a region of interest (R.O.I.) of a patient, etc. in detail.
2. Description of the Prior Art
The NMR phenomenon, which was discovered independently by Bloch and Purcell in 1946, has become, since then, an indespensable analyzing means for analysis of the structure of matter and for other physical and chemical fields.
This NMR phenomenon is a magnetic interaction of the magnetic moment of nuclei with an external field and has characteristics that the magnetic energy is considerably smaller (about 10.sup.-9) than that used for X-ray computed tomography and has almost no influences on the living body, e.g. human body.
The attempt to apply this NMR phenomenon to the imaging was proposed at first by Lauterbur in 1974. Although a number of NMR imaging methods have been developed thereafter, nowadays pulse methods, which are excellent in measurement precision and S/N ratio, are most often utilized. The details of the pulse NMR imaging method is described e.g. in "Pulse and Fourier transformation NMR" written by Farrer and Becker and translated in Japanese by Akasaka and Imoto, Yoshioka Publishing Co. (1979), etc.
All the pulse methods utilized at present are methods by which an object to be inspected is excited by a powerful pulse high frequency magnetic field covering all the Larmor frequencies contained by the object to be inspected and free induction decay (hereinbelow abbreviated to FID) signals produced thereby are analyzed in frequency.
Among the pulse methods mentioned above are utilized the following various methods and their combinations:
(1) Fourier transform method; PA1 (2) projection reconstruction method; PA1 (3) selective excitation method; and PA1 (4) alternating gradient magnetic field method.
Each of these methods described above has both merits and demerits. For example the Fourier transform method and the projection reconstruction method have a problem that measurement time is long, although relatively good image quality can be obtained, and it is basically difficult to promote further speed-up which has been already realized. On the other hand the selective excitation method is characterized in that although image quality is not so good, measurement time is shorter.
In the case where a cross section of an object to be inspected is imaged, it is sometimes desirable not to image a whole cross section, but to image only a particular region of interest with high precision in an enlarged scale. An example for it is the case where it is desired to know variations of an affected part after an operation or medication, etc.
In this case, it is necessary to know previously the position of the region of interest mentioned above. Usually it is conceived to specify its approximate position on the basis of tomographs made beforehand, but in general it is difficult to locate a new image at an affected part which was imaged previously.
Accordingly a method, by which the region of interest is specified on the basis of an image taken just before an enlarged imaging, has been adopted.
However, in this case, since a long measurement time is necessary for the projection reconstruction method, apart from too much fruitless long wait time until a doctor can specify the region of interest, this provokes a problem that the through-put of the apparatus is made worse. To the contrary, by the selective excitation method, since its image quality is not sufficient, satisfactory effect cannot be expected, even if an enlarged image is made.